Systems have heretofore been proposed which utilize the polar Kerr effect to provide a read signal from a magneto-optic storage disk. The polar Kerr effect itself is observed by the changes in the state of polarization of light that occurs after normal reflection from a perpendicularly magnetized medium. In one such system, the p-polarized light from a laser is passed through a polarizing beam splitter to the magneto-optic medium. The light reflected from the medium has both s and p components because of the polar Kerr effect in the medium. The p and s components refer to light vibrating parallel and perpendicular to the plane of incidence, respectively. The phase direction of the s component contains the information about the magnetization direction of the magnetic domains of the medium which represent encoded information. The phase direction of the s component, for example, up or down, can be measured by referencing it to the phase direction of the p component. See "Signal-to-noise ratio for magneto-optic readout from quadrilayer structures" by G. A. N. Connell et al, Applied Physics Letters, 42 (8), Apr. 15, 1983, pp. 742-4.
The s component and the p component of the reflected light are incident upon the aforenoted beam splitter which is commonly composed of an isoleses right-angled prism, the hypotenus face of which is coated with a number of thin-film layers of alternate high and low refractive index materials and then cemented to an identical uncoated prism. Beam splitters are designed to work at a particular wavelength for light striking the front face of the cube at normal incidence, with the optical thickness of each layer equal to a quarter of the design wavelength. Beam splitters so configured give very high reflectance of the s component of the read beam while providing a very high transmission of the orthogonally polarized p component, as shown in FIG. 1. Thus, normal incidence of the read beam on the media-facing face of the beam splitter will not provide a p component traveling in the same direction as that of the s component, and thus the phase or polarization direction of the s component cannot be measured by reference to the phase or polarization direction of the p component.
In one published magneto-optic medium readout system, see the previously referenced Connell et al article, the polarizing beam splitter is rotated about the vertical axis by an angle .theta. such that a small fraction of the p component (and a large fraction of the s component) are permitted to leak into a differential detector arrangement, with the p component providing the needed reference for measuring the phase of the s component. The rotation of the polarizing beam splitter is, therefore, allowing it to behave as a low quality, leaky splitter. However, the beam splitter rotation technique has a problem in that for a given rotation .theta. of the beam splitter, for example, .theta.=10.degree., the optics of the differential detector arrangement must be rotated by 2.theta. or 20.degree., as shown in FIG. 2. The latter rotation consumes too much space in the optics module and hence the rotated polarizing beam splitter arrangement is disadvantageous.